Adding on to this that Barnes and Noble sells DRMed ePUB files that are relatively easy to strip DRM from using Calibre.

So if you can't find a book anywhere else, at least they don't use a proprietary format and still allow you to download your books using their PC software.

I was a semi-early adopter in the ebook space and I have refused to get onboard with the kindle ecosystem from the start. There's no reason for their proprietary format other than complete control over things they pretend to sell you. Amazon is also the Walmart of books and uses their position to browbeat publishers and authors into taking smaller cuts of sales.

One of my friends got a book published and I waited and waited for it to be available anywhere else. Eventually just bought what was probably a print on demand copy from Amazon because that's the only place his publisher sold books. I never buy physical books anymore but I'd rather do that than buy a kindle book.